1. Field of the Invention
The present invention relates to means of limiting reverse fluid flow through conduits. More specifically, the present invention relates to a check valve assembly suitable for employment in a drill string in which drilling fluid or "mud" is being pumped downwardly through the string. The assembly is designed to limit upward flow of fluids when the drill bit enters a high pressure area.
2. Description of the Prior Art
When drilling a well, there may arise a need for a device to prevent the uncontrolled upward flow of the drilling fluid or "mud" in the drill string, e.g., should the drill bit enters a high pressure area. Under normal operating conditions, the device should allow unrestricted downward flow of the mud.
The prior art has suggested a number of valve assemblies designed to allow fluids or effluent materials to be pumped down through a drill string and to prevent reverse or upward flow therethrough. See, for example, U.S. Pat. Nos. 1,577,740 and 1,790,480.
If the valve elements are directly in the flow stream, the materials pumped down through the drill string may erosively wear the valve components, particularly when such materials carry abrasive particles. Previous check valve devices have employed a ball valve member and a seat member, along with a retainer or cage assembly. In these devices, the valve assembly is located directly in the flow stream and therefore subjected to the erosive action of abrasive material in the fluid. Such valves also restrict the downward flow of fluid and, with the valve assembly located directly in the flow stream, it is impossible for equipment to be lowered through the drill string past the assembly.
During drilling operations, the drill string may frequently be removed from the bore for maintenance of the drill bit. The valve assembly should allow fluid to empty from the drill string when it is raised from the bore. It is preferable that the valve assembly also allow fluid to flow at a limited rate upward past the assembly when the drill string is being lowered into the well bore. By allowing the drill string to fill from the bottom, fluid does not have to be pumped in at the top to lower the drill string and to prevent the drill string from collapsing because of pressure differentials. Valve assemblies previously used, either allow no reverse fluid flow, or a predetermined amount of flow at all times. The valve that allows fluid to flow all the time is undesirable. Such a valve works fine when lowering the drill string into the well bore; but, when the drill bit enters a high pressure area, the flow can never be completely stopped.
Other devices have been designed to control only the upward flow of fluid in well tubing and are not designed for use in a drill string, where fluid is allowed in both directions. These devices are used in production strings to shut off the flow of oil if damage occurs to equipment at the wellhead. Many of these devices have a ball valve located in a side pocket out of the flow stream and a movable inner sleeve for displacing the ball from the side pocket when the differential pressure is increased sufficiently.
In the aforementioned U.S. Pat. No. 3,850,191, a new and improved drill string check valve assembly is disclosed which provides an unrestricted flow path for unrestricted downward flow and passage of flowline equipment; but, which is provided with means for regulating the rate of reverse flow so that the drill string can be lowered into the well bore without having to pump fluid into the top of the drill string. In such a check valve, a tubular housing is provided, having a recess in its wall, for normally retaining a ball valve closure member out of the flow stream. Thus, the ball itself doesn't restrict the downward flow of fluid and is at least partially protected from erosion by abrasive material in the fluid. In its preferred form, the closed end of the ball recess may also communicate with the flowbore through a pressure equalizing passage by which the rate of reverse flow can be regulated. Although such a check valve assembly is superior to those of the prior art, the recess, and to some degree, the ball member itself, is still subject to some degree of erosion.